Conventionally, automatic gain variable devices of the above-explained acoustic apparatus have been developed (refer to, for instance, patent literature 1). FIG. 9 is a block diagram for indicating the arrangement of the automatic gain variable device of the conventional acoustic apparatus disclosed in the above-explained patent literature 1.
The conventional automatic gain variable device 90 shown in FIG. 9 is arranged by a signal amplifying unit 91, a signal strength detecting unit 92, a signal strength holding unit 93, and a signal gain setting unit 94. The signal amplifying unit 91 amplifies an input signal “Si” based upon a gain set by the signal gain setting unit 94, and then, outputs the amplified input signal “Si” as an output signal “So.” The signal strength detecting unit 92 detects a signal strength of the input signal “Si” for a predetermined time period. The signal strength holding unit 93 holds a maximum signal strength detection result detected by the signal strength detecting unit 92. The signal gain setting unit 94 sets a constant gain to the signal amplifying unit 91, while the constant gain corresponds to the maximum signal strength held by the signal strength holding unit 93.
In such an arrangement, the signal strength is amplified and adjusted based upon a low gain with respect to a large audio input signal, whereas the signal strength is amplified and adjusted based upon a high gain with respect to a small audio input signal.
Patent Literature 1: JP-A-11-75286